A double servo system has been proposed for use in a hard disc drive (HDD) employed in an information processing apparatus, such as a personal computer.
The double servo system is rather popularly employed in optical disc apparatuses. However, in recent magnetic disc apparatuses, track pitches are narrowed to, for example, below 10 nm. This gives rise to a background of promoting densification more highly than in optical disc apparatuses as described above, to such an extent that conventional control systems cannot respond to the densification.
In particular, popular VCM control employs a great number of filters, such as a lead lag filter (phase advance/lag compensation) and an integration compensator which obtains a gain in a low band and improves an outer disturbance property, in order to stabilize an originally unstable actuator. These filters are indispensable to a VCM. Although stabilization is unavailable without these filters, the filters easily cause a phase delay and put limitations to obtaining of higher gains, and are ineffective for improvement of an outer-disturbance compression property.
There is also a background that, even a double servo system is introduced, VCM control takes over a conventional method in many cases, and therefore, a phase margin becomes finally a bottleneck which leads to unsatisfactory results from a viewpoint of high gains.